Quantifying vegetation response to environmental changes on the Galapagos Islands, Ecuador using the Normalized Difference Vegetation Index (NDVI)

Abstract

The vegetation of the Galapagos Islands (Ecuador) is strongly influenced by climate. El Niño events, seasonality, isolation, volcanism, and increasing human activity define the ecosystems of the archipelago. Given their socio-cultural and economic importance, it is critical to monitor the response of Galapagos vegetation to changes in climate and assess its vulnerability. This study explores the potential to use Normalized Difference Vegetation Index (NDVI) as a proxy to describe trends in primary productivity in the Galapagos (2000-2019) and models the relationship between NDVI and climate variables including evaporation and atmospheric carbon dioxide concentration. From numerous possible co-variates compiled from reanalysis and satellites, we identify the independent variables that most strongly influence NDVI using the least absolute shrinkage and selection operator (LASSO) method. Significant variables, including carbon dioxide concentration, evaporation, and autocorrelation (1-month and 12-months lagged NDVI) are then used to model NDVI in a generalized linear model (GLM) framework. The model predicts NDVI more effectively where values for NDVI are high (high elevation, lush vegetation), and clearly reflects seasonality. Validation of the model across pixels produces R2 values ranging from 0.05 to 0.94, and the mean R2 is 0.57 (0.65 for elevation >20 m). This methodology has the potential to continuously and non-intrusively monitor vegetation changes in sensitive ecological regions, such as the Galapagos.